Noise in electrical systems and other types of systems such as electro-optic and electro-acoustic may disrupt both the amplitude and phase of signals. However, because many systems are relatively insensitive to fluctuations in amplitude, the fluctuations in phase (denoted as phase noise) are generally more problematic. For example, an oscillator may be designed to output a sinusoid at a desired frequency. Oscillators typically include some type of amplitude-limiting feature so that only phase noise will be a major noise contributor to the output sinusoid.
Because phase noise is such an important factor of overall noise, designers often desire a measure of the phase noise for a given system. Various approaches have been used to characterize phase noise. For example, amplifiers have been characterized by inputting a signal of known frequency into the amplifier and measuring a resulting amplified output in a spectrum analyzer. But the sensitivity of such an approach is limited by the relatively-poor sensitivity of the spectrum analyzer. Moreover, it is difficult to measure phase noise at frequencies close to the carrier frequency.
Unlike a spectrum analyzer, a phase-locked discriminator system has relatively good sensitivity and allows measurements close to the carrier frequency. However, the configuration of a phase-locked discriminator system is cumbersome and time consuming. Thus, an automated phase-locked discriminator noise test measurement system has been developed as described in U.S. Pat. No. 6,393,372 that alleviates the cumbersome nature of such systems. FIG. 1 illustrates an embodiment of such an automated system 1. A low-noise source 9 provides an input signal 11 for driving a unit-under-test (UUT) 3. UUT 3 may be any device for which a user desires a phase noise test measurement such as an amplifier, phase-shifter, diplexer or other suitable device or system of devices. UUT 3 receives the input from source 9 and processes it to provide an output signal 5. For example, if UUT 3 is an amplifier, output signal 5 would be an amplified version of input signal 11. Output signal 5 is amplified by variable amplifier 15 to provide an input signal 23 to a mixer 21. Source 9 also provides a version of input signal 11 to a variable phase-shifter 29. Variable phase-shifter 29 shifts input signal 11 by 90 degrees to provide a phase-shifted signal 25 to another input port of mixer 21. In this fashion, the “carrier” signal (input signal 11) is eliminated from a mixer output signal 41. To keep output signal 41 in the proper dynamic range of an analog-to-digital converter (ADC) 49, mixer output signal 41 is processed by a low-noise matching amplifier 43 to provide an output signal 42 to ADC 49.
To eliminate the carrier signal, the phase-shifted signal 25 must be in quadrature (shifted 90 degrees) with respect to the carrier. If quadrature is not established, a DC offset will be present in a digital output 44 from ADC 49. A processor 55 monitors digital output 44 and controls phase-shifter 29 using a control signal 65 to maintain quadrature. The elimination of the carrier signal from low-noise source 9 also depends upon whether the carrier (input signal 11) and the phase-shifted version of the carrier (signal 25) are of equal power when entering mixer 21. Thus, analogous to the control of phase-shifter 29, processor 55 also controls variable amplifier 15 responsive to processing digital signal 44 using a control signal 67 to maintain equal powers for signals 25 and 23. These powers need not be maintained exactly equal but instead may merely be within a sufficient range of each other so that linear operation of mixer 21 is assured. Those of ordinary skill in the art will appreciate that variable amplifier 15 does not just amplify but may also attenuate responsive to control signal 67. For example, if UUT 3 is an amplifier, variable amplifier 15 will have to attenuate output signal 5 to keep signals 23 and 25 in comparative power equality. Processor 55 may also control low-noise matched amplifier 43 using a control signal 71 to maintain signal 42 in the proper dynamic range for ADC 49.
Having controlled the components for quadrature operation, processor 55 eliminates the carrier from digital output signal 44 from ADC 49 such that digital output signal 44 simply represents the phase noise. The phase noise injected by low noise source 9 may be accounted for by a calibrating operation such that UUT 3 is removed and source 9 simply feeds amplifier 15 directly, although such a direct feed may occur through a delay line (not illustrated). The resulting phase noise in digital signal 44 during calibration may be stored in a memory associated with processor 55. Thus, during testing of UUT 3, processor 55 (or a spectrum analyzer associated with processor 55) may perform a fourier analysis of digital signal 44 to determine the phase noise power. The measured phase noise may then be adjusted by the phase noise injected by source 9 to determine the additive phase noise supplied by UUT 3.
The phase noise measured in digital signal 44 depends upon the frequency of input signal 11 provided by source 9. For example, UUT 3 may be quite noisy at one frequency but less so at another. To measure phase noise across a range of frequencies, processor 55 may command source 9 to change the frequency of input signal 11 using a command signal 69, measure the resulting phase noise, change the frequency again, measure the resulting phase noise, and so on. Advantageously, such measurement is performed automatically and accurately with no manual intervention or tuning as would be necessary in conventional phase noise test measurement systems.
Although a phase-locked discriminator system 1 represents a dramatic advance in the art, certain challenges remain. For example, suppose UUT 3 itself is a fiber optic link. A conventional fiber optic link 200 is illustrated in FIG. 2. An amplifier 201 amplifies an electrical input signal sin(t) to drive a laser diode 205. In turn, laser diode 205 drives an optical signal into an optical fiber 210. After passing through optical fiber 210, the optical signal is converted into an electrical signal 220 in a photodetector 215. An amplifier 230 amplifies signal 220 to provide an output signal sout(t). Many factors are involved in properly biasing link 200 for optimal performance. For example, matching amplifiers 201 and 230 to the link, the biasing of transistors within amplifiers 201 and 230, and the biasing of laser diode 205 and photodetector 215 are all factors that affect the additive phase noise that link 200 injects into the output signal sout(t). However, a designer of link 200 has no intelligent way of setting these factors. A similar situation exists for the proper setting of variables in many other systems and devices.
Accordingly, there is a need in the art for improved techniques to properly set variables in systems and devices so as to minimize phase noise in these systems and devices.